30-day weather outlook for September 10, 2018 – October 10, 2018
Summary- A longwave trough continues to bring mild or cool conditions across N and central California from Sept 13-22nd.
Strong upper high pressure over the southern Rockies and SW Great Plains will maintain the hot conditions into S and SE California. High pressure will tend to build or rebuild westward into central California and the SierraNV.
Warmer than normal sea surface will continue near the coast of central and S California and western Mexico. Sea surface temperature on the evening of Sep 10 was 68 to 72 in the inner SOCAL Bight, and 63-64 off Pt Arguello.
Dry, or mainly dry, conditions continue through the end of Sept. Our cfsdailyAI system is showing rains from the beginning of Oct through about the 12th in mostly the SierraNV and S California. Heaviest and most widespread rains occur Oct 10-12. These are preceded by another rain during Oct 1-4th, focusing in S California.
For temperatures, CFSDailyAI system suggests hottest days around Sept 29-30, and a warm period from Oct 8-9. Mild nights and clouds with some showers occur most of the period from Oct 1-10. The 11-12th is sharply colder at night but continued mild days, suggesting a dry and mild early fall period, with a distinct trend into clear cold nights and sunny dry warm days of fall.
The El Niño (warmer than normal sea surface) along the equatorial central and east Pacific, continues to be moderate through the fall. The time of normal maximum intensity of El Niño is generally Dec through Feb. Sea surface temperature anomalies (SSTA) remain high through the fall and into early winter. The fall and early winter correspondingly are also warm, per CFSv2 model guidance.
The sea surface temperature anomaly pattern off South America at 30-45S supports a possibly aggressive frost period this early to mid-spring in Chile’s viticultural areas, from Santiago south. This includes Vicuna, Elke Valley Valparaiso and the intermediate valleys (Bernardo-O-Higgens area). This risk includes the more critical period…spring blooms for winegrapes in late Sept and all of October.
In the Near Term – Sept 15-29…Salinas Valley-San Luis Obispo Co…Troughing and a cool period continue through Sept 17, then it turns warmer from the 18-22nd. The hottest days occur from the 28-30th. Other than coastal drizzle at the beaches, and coastal foothills, rain is not indicated.
…S Calif Avocado Area, San Luis Obispo Co to San Diego Co…Sea surface temperatures along the SOCAL coast have continued to be unusually warm. This will tend to maintain the mild nights in SOCAL, even if days near the coast are less warm, with 70s for the most part near San Diego, and upper 60s in Ventura Co. This will tend to maintain warm and humid nights. Leftover moisture from tropical cyclones is still possible, but not currently indicated until Oct 1-11.
Summary – Sept 30 – October 15… In the southern California avocado growing areas, from San Luis Obispo south, hot days are possible on Sept 29-30. On Oct 1-2, there appears an increased chance for showers as an upper low draws tropical moisture north into SOCAL, appearing as a deep marine layer type pattern with a drizzly tropical rain, similar to a period of Hawaiian trade-wind showery rains with embedded drizzle. A deeper frontal zone with tropical moisture comes N into SOCAL mountains, and possibly the desert areas, to the Tehachapi’s and S Sierra on Oct 3rd and 4th. A similar rainy period arrives on the 10-13th with some periods of showers (per current scenario). The rains appear most likely on the 9-10th, and into the SierraNV on 11-12th.
Seasonal Outlook / El Niño Update...Oct 15 – Dec 31… It appears that the winter rains may be slow to arrive in Nov – Dec in central and N California. Above normal temperatures will be predominant, suggesting continued high snow levels. This makes logical sense due to the expected warmer than normal sea surface off to the southwest of NORCAL.
El Niño-associated warming of the equatorial sea surface in the central and east Pacific to near Peru/Ecuador is occurring and will continue. However, despite the occurrence of El Niño, it will be difficult to realize significant winter-type rains until after the middle of January 2019.
For November, there continues the trend to the return of warm and dry conditions, per the latest CFSv2 guidance. December continues similarly dry, except for the last week of Dec around the holidays.
For S California, from Oct 15– Dec 31…Near normal rainfall is currently suggested for the latter part of Oct as high pressure returns for a period. For Nov and Dec, it appears mostly dry as high pressure continues to dominate, as upper lows remain further to the west. Fronts approaching from the west will intensify upon approaching the coast, then lose energy upon arrival at the coast.
After Nov 1, the next opportunity for rains in SOCAL is often during Dec 12-20th. Currently, it appears mostly dry in SOCAL. NORCAL also appears dry and warmer than usual, but with rains during the holidays.
...Alan Fox, Fox Weather LLC...